Well, the first thing we can observe is that we're not getting attacks by these highly nonsuspicious types, so we can safely assume that for whatever reason they're not cost-effective (we do know that in fact nonsuspicious types get much less screening than suspicious types, so, theoretically, they should be 100% of attackers).
The real beauty of this logic is that it doesn't actually matter if it's correct. If it is, great -- we need to harass suspicious types even more than we already are. But maybe it isn't. If terrorist groups are sending suspicious attackers for irrational or idiosyncratic reasons... that doesn't matter to us! We're trying to defend against the people they do send, not the people we think they should send. So we need to harass suspicious types again.
That said, there are plenty of heavy costs associated with kidnapping foreigners from around the world and using them as hostages:
1. The language barrier. If you want to coerce someone, you need to be able to make them understand what you want them to do.
2. Security. Your target country will hate you and make great efforts to root you out. These groups survive in countries where they have popular support. Pakistan might be willing to look the other way while you hang out and make trouble for the US; they're much less likely to look the other way while you kidnap and threaten to kill Pakistanis.
3. Public relations. Again, these groups survive where they have a certain level of popular support. But the same populace that doesn't really care when bad things happen to the Great Satan might not feel the same way about randomly kidnapping and killing bystanders from around the world.
4. Morale. The internal reflection of public relations. Most of the people in the group are there because they think it's the right thing (or can be talked into it). Kidnapping and killing peaceful foreigners from around the world could be a blow to that. Telling 80-year-old women to kill themselves just doesn't feel like the right thing.
"Well, the first thing we can observe is that we're not getting attacks by these highly nonsuspicious types, so we can safely assume that for whatever reason they're not cost-effective"
[citation needed]
For example, there were plenty of IRA bombings by older folks, etc.
Even if you only consider airplanes, there have been hijackings, suspected bombings, etc by older folks.
Just because today's media focuses on certain ethnotypes and attacks does not make them actually predominant in actual attacks.
The real beauty of this logic is that it doesn't actually matter if it's correct. If it is, great -- we need to harass suspicious types even more than we already are. But maybe it isn't. If terrorist groups are sending suspicious attackers for irrational or idiosyncratic reasons... that doesn't matter to us! We're trying to defend against the people they do send, not the people we think they should send. So we need to harass suspicious types again.
That said, there are plenty of heavy costs associated with kidnapping foreigners from around the world and using them as hostages:
1. The language barrier. If you want to coerce someone, you need to be able to make them understand what you want them to do.
2. Security. Your target country will hate you and make great efforts to root you out. These groups survive in countries where they have popular support. Pakistan might be willing to look the other way while you hang out and make trouble for the US; they're much less likely to look the other way while you kidnap and threaten to kill Pakistanis.
3. Public relations. Again, these groups survive where they have a certain level of popular support. But the same populace that doesn't really care when bad things happen to the Great Satan might not feel the same way about randomly kidnapping and killing bystanders from around the world.
4. Morale. The internal reflection of public relations. Most of the people in the group are there because they think it's the right thing (or can be talked into it). Kidnapping and killing peaceful foreigners from around the world could be a blow to that. Telling 80-year-old women to kill themselves just doesn't feel like the right thing.